This invention relates to a high strength gear, a power transmission mechanism using the high strength gear and a production method for the high strength gear, and more particularly to the high strength gear which used as a gear of a mechanical element of a variety of mechanical structures such as an automotive vehicle, an industrial machine, an agricultural machine and the like.
It is expected in the future that motor-driven automotive vehicles increase from the viewpoint of coping with environmental problems. Gears of a speed-reducing device used in the motor-driven automotive vehicles are required to reduce a rotational speed (reaching several ten thousand r.p.m.) of an electric motor to a certain low rotational speed through their size is maintained compact. Accordingly, there is a fear of bringing about a surface damage such as seizure and wear of sliding members due to a temperature rise at sliding surfaces.
As a measure of preventing such a surface damage, coating the surface of the sliding members with diamond-like carbon (referred hereafter to as “DLC”) has been recently used. The DLC includes bondings of diamond and graphite in a mixed state and macroscopically has a structure seemed amorphous. Accordingly, the DLC has a characteristics intermediate between diamond and graphite and is excellent in hardness and solid lubricating characteristics. As a result, it is expected that the DLC is useful for lowering a friction loss and preventing seizure and wear due to heat generation under sliding, also in CVTs (Continuously Variable Transmissions) with gears and belts.
However, there may arises a problem that the DLC film peels off owing to an internal stress generated under a difference in thermal expansion between the DLC film and a material to be treated and owing to shortage in toughness. This problem becomes serious as the thickness of the DLC film increases.
In view of the above, it has been proposed to laminate a layer of metal or semimetal such as titanium (Ti), zirconium (Zr), chromium (Cr), tungsten (W) or the like on a carbon layer so as to improve the toughness of the carbon layer, as disclosed in Japanese Patent Provisional Publication No. 11-1013.
Additionally, it has been proposed to laminate a high carbon density layer and a low carbon density layer thereby improving the toughness and improving a friction characteristics under a solid lubricating action of abrasion powder in the low carbon density layer, as disclosed in Japanese Patent Provisional Publication No. 2002-322555.
Further, it has been proposed to improve a seizure resistance and a friction characteristics of a sliding member in a lubricating oil by using a DLC film which is obtained by a plasma CVD process and contains 20 to 40 atomic % (referred hereinafter to as “atm %”) and 1 to 5 atm % of silicon, as disclosed in Japanese Patent Provisional Publication No. 2003-336542.
Furthermore, it has been proposed to improve a friction characteristics of a sliding member in an engine oil by using a DLC film formed by an ion plating process and contains not more than 10 atm % of hydrogen, as disclosed in Japanese Patent Provisional Publication No. 2000-297373. This seems to be accomplished under the contribution of a grinding effect of particle materials called droplets, of a target material for forming a DLC film. The droplets are unavoidably produced in the ion plating process.
Concerning the above proposals or techniques of Japanese Patent Provisional Publications Nos. 11-1013 and 2002-322555, the improvement in the characteristics is exhibited under no lubrication, and therefore the Publications never discuss the characteristics in a lubricating oil.
Concerning the above proposal or technique of Japanese Patent Provisional Publication No. 2003-336542, although a remarkable friction reduction is exhibited, plasma becomes unstable in case that a film formation is accomplished in the atmosphere of hydrocarbon-based gas particularly by an ion plating process. This causes a misfire during a treatment and therefore there is such a problem as to make it impossible to conduct a stable film formation. Particularly in case that an unevenness is formed like teeth of a gear, a film is difficult to be formed at a concave-shaped base section of the gear as compared with a convex-shaped tip section, and therefore the thickness of the film unavoidably increases throughout the whole gear in order to obtain a certain film thickness also at the base section. This raises a problem of the gear being lowered in peel strength owing to the thickened film, and a long time is required for the film formation thereby lowering stability of the film and making it impossible to select a process of the film formation.
Thus, in the above conventional techniques, it has been difficult to form a DLC film on a gear which film is low in friction in a lubricating oil and cannot peel off even in case of being used under a high bearing pressure and under a high speed sliding.